Ice build-up on external aircraft structures is extremely dangerous in that it not only adds weight to the aircraft, but also it distorts the profile of lifting surfaces and impairs the function of control surfaces such as rudders, elevators and ailerons. Also, ice forming on external airspeed indicators and the like have often caused total loss of aircraft with passengers and crew, due to loss of air speed and altitude indication.
Also, during flight under auto-pilot, the forces building on the control surfaces due to unknown accumulations of ice can cause excessive forces on the control surfaces, so that the auto-pilot servo system cannot continue to respond, and it may fail, adding excessive and/or unanticipated forces abruptly to the control yoke, possibly resulting in a crash.
A considerable body of prior art has been developed over the years to provide reliable icing indicators for aircraft.
As an example, U.S. Pat. No. 4,333,004 shows an arrangement including sensor and reference wires directly exposed to the airflow past a vehicle. The wires are electrically conductive, and each has a resistance varying with its temperature. A continuous fixed voltage is passed through the reference wire. A short pulse of greater constant voltage is passed through the sensor wire, the voltage being of sufficient magnitude to cause the rapid heating of the sensor wire during the pulse. The resultant rate of heat of the sensor wire, and its final temperature at the end of the pulse, is determined by whether ice or water is on the sensor wire. Circuitry is provided to generate an ice-present signal when the difference in current flow during the pulse through the sensor wire and through the reference wire is of such magnitude and sign as to indicate that ice was in contact with the sensor wire. Due to water's heat of fusion, the effect of ice on the sensor wire is readily distinguished from the effect of water.
U.S. Pat. No. 4,553,137 shows an improved ice detector having a vibrating element which has a front portion and vibrating means suitably disposed with respect to the vibrating element for exciting the vibrating element into vibration and means for sensing a shift of the frequency of vibration of the vibrating element resulting from a change of mass of the vibrating element due to ice accumulating thereon. The improvement is characterized in that the ice detector is supported with respect to a surface exposed to an air stream such that a cap on which ice forms and which is disposed on the front portion of the vibrating element is exposed to the air stream and is formed to be substantially conformal to such surface.
U.S. Pat. No. 4,461,178 shows a system for the detection of wing icing by monitoring variations in flexural waves transmitted through the outer plate material of an aircraft airfoil. The flexural waves in the plate of the wing airfoil are more subject to variation from the accumulation of ice on the wing than the compressional waves. The flexural waves are detected apart from the compressional waves, which tend to remain relatively constant, to provide an indication of icing. Changes in amplitude, phase or dispersion characteristics of the flexural waves are detected to indicate ice buildup, and, in one embodiment, these values are ratioed to corresponding levels in the compressional wave in order to provide compensation for variations other than ice buildup. Ultrasonic waves may be coupled directly from a transducer to the airfoil plate or via an ultrasonic waveguide interposed between the transducer and the plate. The receiver for the ultrasonic waves to be detected may be positioned to receive direct flexural waves transmitted over a distance through the plate or flexural waves reflected from reflecting boundaries in the plate.
Other patents directed to overcoming the problem of detecting icing on aircraft are listed in the enclosed listing of prior art.
The known art suffers from various drawbacks such as high cost, complexity, weight, high current drain, an inability to differentiate between rime and clear ice and generally lack of dependability in indicating ice formation, especially at the early phases of ice formation when it is extremely critical for a pilot to be warned while there is still time to take evasive action.
It is accordingly an object of the present invention to provide an aircraft icing sensor which overcomes the drawbacks of the known devices of this type, and which is especially directed to providing aircraft sensing apparatus that is of light weight, low complexity, a high degree of dependability and has low power drain and is ergonomically suited for an environment such as an aircraft cockpit.
Further objects and advantages of this invention will be apparent from the following detailed description of a presently preferred embodiment, shown schematically in the accompanying drawings.